It is well-known to mount a timber-working head, for example in the form of a harvesting head, to a forestry work machine to perform a number of functions in connection with timber. Such heads may be used to grapple and fell a standing tree and process the felled tree by delimbing, possibly debarking (depending on the configuration of the head), and cutting the stem of the tree into logs of predetermined length.
Processing the felled tree typically involves feeding the resulting stem along its length relative to the head using a feed mechanism. One well known system uses arm mounted hydraulic rotary drives having a feed wheel at the end of each arm. The arms may be driven by hydraulic cylinders to pivot relative to the frame of the head in order to grapple the stem with the feed wheels, which may then be driven in the desired direction. In order to ensure that the stem is firmly grasped in the desired centre position, a mechanical link between the arms is used so they open and close together.
However, the stems processed by the harvester head may be ill-formed, for example having sweep (i.e. variation in the axial linearity of the stem) or other contour irregularities (e.g. bulges, depressions, lack of circularity). In such cases the fixed relationship of the arms relative to each other means that one feed wheel may have a different degree of contact with the stem surface than the other, impacting on traction. This can cause hydraulic oil to bypass, leading to one wheel slipping and spinning—causing less than optimal feed performance and potentially damage to the stem by the slipping wheel ripping into the surface. There can also be further ramifications in terms of damage to the motors themselves as the result of this slipping and motor cavitation.
The mechanical link also adds weight and complexity to the harvester—particularly in the steel support frame and pins required for pivotal connection—which in turns adds to the cost of manufacture. These also create potential points of mechanical failure, particularly where ill-formed stems lead to an imbalance of forces being applied to the two arms—essentially attempting to rip them apart. As the operation of the feed arms is a crucial component of many, if not all, of the functions of a harvester head, time required to repair the link may reduce the productivity of the forestry work machine.
All references, including any patents or patent applications, cited in this specification are hereby incorporated by reference. No admission is made that any reference constitutes prior art. The discussion of the reference states what their authors assert, and the applicants reserve the right to challenge the accuracy and pertinence of the cited documents. It will be clearly understood that, although a number of prior art publications are referred to herein, this reference does not constitute an admission that any of these documents forms parts of the common general knowledge in the art, in New Zealand or in any other country.
Throughout this specification, the word “comprise” or “include”, or variations thereof such as “comprises”, “includes”, “comprising”, or “including” will be understood to imply the inclusion of a stated element, integer or step, or group of elements integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.
Further aspects and advantages of the present disclosure will become apparent from the ensuing description which is given by way of example only.